The invention relates to a method for locating partial discharges in an electric high-voltage apparatus, such as a power transformer, which apparatus is arranged in a container filled with an insulating medium, in which
a number of ultrasonic detectors are disposed in different detector positions on the outside of the container and in acoustic contact therewith,
the times of arrival for an acoustic wavefront arriving at the detectors and originating from a partial discharge are determined, and
from the times of arrival and the distances between the detector positions, information for locating the partial discharge is derived.
The invention also relates to a device for locating partial discharges in an electric high-voltage apparatus, such as a power transformer, which is arranged in a container filled with an insulating liquid. The device comprises a number of ultrasonic detectors adapted to be disposed outside the container and in acoustic contact therewith, and time measuring means connected to the detectors for determining the arrival times for an acoustic wavefront arriving at the detectors and originating from a partial discharge.
It is previously known that partial discharges (referred to as xe2x80x9cPDsxe2x80x9d in the following) in electric high-voltage apparatuses, such as large power transformers, are associated with emission of ultrasound. It is also known to detect and to attempt to locate PDs with the aid of ultrasonic detectors. Two methods for location are described in Phung, James, Blackburn, Su: xe2x80x9cPartial Discharge Ultrasonic Wave Propagation in Steel Transformer Tanksxe2x80x9d, 7th International Symposium on High voltage Engineering, Technische Universitxc3xa4t Dresden, Aug. 26-30, 1991, pp. 131-134.
In a first method, there is used, for example, an ultrasonic detector which is moved between different positions on the outside of a transformer tank. In each position, the time of a PD is detected electrically, and the time of the arrival of the corresponding acoustic signal at the detector is detected with the aid of the ultrasonic detector. In this way, assuming a linear propagation with a known velocity, the distance from each detector position to the current PD is obtained. With the aid of the thus obtained distances from, for example, four detector positions and with the aid of the measured detector positions, the position of a PD may be determined by triangulation. This method is also known from U.S. Pat. No. 3,622,872.
In a second method, the difference in time between the arrival of the acoustic wavefront at two ultrasonic detectors arranged at different locations is used. Assuming a linear propagation with a known velocity, the measured difference in time together with the detector positions defines a hyperboloid which constitutes the locus of the detected PD. By using an additional pair of detectorsxe2x80x94or by moving the same detector pairs to new positionsxe2x80x94a second hyperboloid may be determined, which together with the first hyperboloid enables location of the PD.
However, the accuracy of location of these known methods has been very low. The position determination methods used require long base lines, that is, large distances between the detectors, to obtain a reasonable accuracy. The propagation of the acoustic waves from a PD to a sensor is, however, of complicated nature. The acoustic waves pass through different materialsxe2x80x94oil, insulating material, windings, the wall of the transformer tankxe2x80x94with different acoustic velocities, and the waves therefore do not generally propagate rectilinearly with a certain constant and known velocity. The acoustic waves from a PD to a number of detectors placed at various locations and at a relatively large distance from one another will therefore, with these known methods, generally have completely different propagation routes, which is the reason for the low accuracy of the known methods of location.
Owing to the requirement for long base lines, it is not practically possible, with these known method of location, to arrange the detectors in a common measurement device. These methods therefore require a movement of a detector or a detector pair between a plurality of different measurement positions, or alternatively, the arrangement of a larger number, for example three or four, of detectors in known positions. This makes the methods slow and complicated and they require measurement of each measurement position relative to the other measurement positions. Further, the calculations are complicated since, during each measurement, the calculations have to be based on the detector configuration which is used at that particular time.
U.S. Pat. No. 3,728,619 describes a further method, wherein two or four detectors are used, which are mounted in a common measurement device and rotatable in pairs. The detectors in one pair are rotated until the phase difference between the signals received by the detectors becomes zero, which gives the direction to the PD. Thereafter, the distance may be obtained by rotating the detector pair through a known angle and observing the phase difference between the signals. This measurement device becomes complicated and clumsy because the detectors must be rotatable in pairs and because of the requirement for good acoustic coupling between the detectors and the walls of a transformer tank or the like. Further, the measurement will be slow and requires a larger number of repeated PDs, which reduces the usefulness of the device in practice.
The invention aims to provide a method and a device of the kind mentioned in the introductory part of the description, which make possible simple, rapid and accurate location of partial discharges in electric high-voltage apparatus.
What characterizes a method and a device according to the invention will become clear from the appended claims.
In a method and a device according to the invention, one single measurement fixture with at least three detectors fixedly mounted thereto is used, and the directional determination is made by determining the difference between the arrival times at the detectors for an acoustic wavefront caused by a PD.
In this way, the complicated and time-consuming positioning and position measurement of a plurality of separate detectors are avoided, which results in a rapid and simple measurement method. The fixed detector configuration further permits the use of significantly simpler and, if desired, preprogrammed equations.
Since the detectors are fixedly mounted in a measurement fixture, this is simple in design and handling and hence economically advantageous. Further, no successive adjustment of detector orientations is required, and hence, in principle, only one single PD is required for a position determination.
The directional determination with the aid of the time differences between the arrival of a wavefront at the detectors, used in the method according to the invention, makes possible a very accurate measurement also with a short base line. The measurement fixture can therefore be designed compact and simple to handle without lowering the requirements for measurement accuracy.